Early Embryonic Development Flashcards
_______ increase number of cells in the embryo
celavage or mitotic cell divisions
in a blastomere, stage is determined by
number of cells
what determines pattern of cleavage
yolk volume
what is yolk
protein and lipids
isolecithal ova type occurs in
most mammals (placenta/marsupials)
isolecithal characteristics
small amount of yolk, evenly distributed throughout cytoplasm
holoblastic cleavage
holoblastic cleavage
blastomeres divide completely
megalecithal animals include
egg laying mammals (platypi, echidna, spiny anteater), birds, reptiles
megalecithal characteristics
abundant yolk with embryo forming cytoplasm at the opposite end of zygote
megalecithal animal pole
embryo cytoplasm
megalecithal vegetal pole
yolk rich cytoplasm
what is meroblastic cleavage
cleavage initiated at animal pole, but large yolk mass does not divide (cell division incomplete)
meroblastic cleavage: blastodisc
small region of animal pole becomes embryo
meroblastic cleavage: yolk
food supply for developing embryo
1st cell division produces
2 daughter cells called blastomeres; 2 polar bodies visible
2nd cell division produces
4 cell stage (4 blastomeres)
3rd cell division produces
8 cell stage (8 blastomeres)
what happens to zona pellucida during these cell divisions?
it remains intact and the same size (does not increase in size as the embryo grows)
as long as zona pellucida remains, what happens to cell sizes during each cleavage
cell size decreases
what is a morula
16-32 cell stage; solid mass of cells
is the zona pellucida intact in a morula?
yes
what is the range of the length of time post-ovulation to reach uterus
2-8 days
most species time to reach uterus post-ovulation
3 days
length of time in dog for post-ovulation to reach uterus
8 days
length in time of mare for post-ovulation to reach uterus
5-6 days
what are the factors that determine the length of time post-ovulation to uterus
amount of smooth muscles in uterine tube
function of fimbrae and cilia
narrowness at ampulla/isthmus junction and uterine tube/uterine horn junction
hormone influence
what does estrogen due to timing post-ovulation to reach uterus
slows
what does progesterone due to timing post-ovulation to reach uterus
stimulates
the longer it takes to reach uterus,
the more number of cleavages are expected and therefore later stage embryo
dog stage when reach uterus
blastocyst
feline stage when reach uterus
morula
equine stage when reach uterus
morula
cattle stage when reach uterus
morula
goat stage when reach uterus
4-8 cell stage
sheep stage when reach uterus
4-8 cell stage
swine stage reach uterus
4-8 cell stage
what transforms the morula to a balstocyst
a fluid filled space or cavity (blastocoele)
trophoblast
outer cell layer, primary source fetal components of placenta
inner cell mass or embryoblast
embryo proper
uterine milk or histotroph is
secretions of the uterus (enzymes, growth factors, cytokines, hormones, proteins, etc.)
what is the function of uterine milk
nourish early embryo until fetal membranes form
zona pellucida prevents _____
implantation to protect embryo from female immune system early in development
what is blastocyst hatching
rupture/disintegration of zona pellucida
what causes weakening of zona pellucida
enzymes by trophoblast and in uterine secretions
rupture of zona pellucida allows _____
release of embryo
occurs about day _____
4-8
embryo remains ____ until attachment occurs
free-floating
transuterine migration
movement of early embryo inside lumen of uterus
cow and ewe transuterine migration (with single embryo)
minimal
ewe with 2-3 embryos that start in same horn transuterine migration
significant
bitch, queen, sow transuterine migration
some
mare transuterine migration
very extensive, until day 16 or 17
why is transuterine damage so extensive in the horse
moves from uterine horn to horn and body several times a day
part of signal to prevent leutolysis and maintain pregnancy
most pregnancies in the hores (66%) eventually implant where?
right uterine horn
what is implantation
process whereby a fertilized ovum becomes implanted in the lining of the uterus of placental mammals
embryo stages prior to implantation are hardy due to
zona pellucida
attachment of trophoblast cells to uterine mucosa can be either _____ or ______
nondeciduate or deciduate
nondeciduate is
noninvasive
nondeciduate occurs mostly in what species
large domestic species
maternal and fetal tissue in nondeciduate
loosely apposed (apposed placenta)
separation easily achieved at parturition without _____
damage to maternal tissue
deciduate (interstitial) is
invasive (destroys uterine mucosa)
what species have deciduate implantation
carnivores, rodents, primates
deciduate animals tissue
maternal and fetal tissue intimately fused (conjoined placenta)
at birth what happens to maternal tissue
maternal tissue is lost
bilaminar embryo; trophoblasts move laterally to expose _____
inner cell mass
inner cell mass flattens to a circular plate consisting of
2 layer—>embryonic disc
what are the two layers making up the inner cell mass
epiblast and hypoblast
what is the epiblast
outer layer, next to trohpoblast cells
what is hypoblast
inner layer, lines inside of epiblast and trophoblast cells
what is gastrulation
process by which bilaminar embryo is converted into a trilaminar embryonic disc, creating 3 primary (primitive) germ layers
what are the 3 layers
ectoderm, mesoderm, endoderm
ectoderm
exterior most
mesoderm
middle
endoderm
internal most
primitive streak
caudal marginal zone of epiblast induces fomration; extends caudal —–> cranial
ectoderm color coding
blue
mesoderm color coding
red
endoderm color coding
yellow (us) green (europe)
primitive node
increased concentration of cells found at the cranial end
primitive streak establishes
craniocaudal axis of developing embryo, median plane, and right-left patterns
initiation site for gastrulation
migration of epiblast cells toward primitive streak
cell ingress to end up between epiblast and hypoblast
epiblasts give rise to all 3 primative germ layers for the embryo
ectoderm
nervous tissue, epidermis
mesoderm
skeleton, muscle, connective tissue, circulatory system, urinary and much of reproductive system
intraembryonic mesoderm
body of embryo (from epiblast)
splanchnic
viscera
somatic
body wall
extraembryonic
outside embryo, embryonic membranes (from hypoblasts)
endoderm
digestive glands, GI epithelium, respiratory epithelium
intraembryonic
body of embryo
extraembryonic
outside embryo, embryonic membranes
what is a notochord
dense column of mesodermal cells on midline, formation initiated at primitive node
notochord becomes a reference axis for
developing embryo
what is the driving force for growth and development
cell signaling
cell growth
increase in cell size
cell proliferation
mitosis to increase cell numbers
induction
one group of cells stimulates (induces) differentiation of an adjacent group of cells
differentiation
a cell or group of cells progressively develop specific structural and functional roles based on their physiological and biochemical attributes
what is patterning
establishment of programmed subsets of cells in proper relationship to each other and surrounding cells and tissues. Necessary for normal tissue and organ formation
what is migration
cells move from one area of the embryonic (or fetal body) to a different area
cell death (programmed cell death)
cells removed in a precisely regulated manner; genetically controlled
what are twins
two individuals which develop in the same pregnancy in animals that are normal monotocous (bear a single offspring)
polytocous
bear many offspring in a single pregnancy
twins in sheep and goats
is the goal!
twins in cattle ranges from _____ to ____ of births
3 to 9%
cattle twins increases with what
assisted reproduction technologies (hormone treatments, embryo transfer)
when twins naturally occur in cattle, it si typically _____
dizygotic
horse twins is most common cause of
non-infectious abortion
twin blastocysts during transuterine migration
typically one is lost
horse twins when both implant….
typically both abort, live birth extremely rare
horse twins when born
high neonatal mortality due to immature development
dizygotic twins result from
two different ova (usually developed from separate follicles) and each is fertilized independently and each embryo develops independently
dizygotic twins has _____ zygotes and ____ genotypes
different; different
in dizygotic each has what
own set of fetal membranes, but membranes and fetal vessels can fuse allowing exchange of blood between fetuses
are dizygotic twins hereditary
yes
monozygotic twins results from
a single zygote that separates into 2 zygotes in early cell division
monozygotic twins are ______ zygotes and ______ genotype
2 different zygotes; 1 genotype (first zygote splits early on when cells could become anything)
monozygotic embryos each have
own set of fetal membranes
duplication of inner cell mass (bilaminar embryo)
- short fetal membranes separate (aminon) others shared (yolk sac and chorion (chorioallantois)
bilaminar embryo documented in
sheep and pig, but not cattle, and 70% of human monozygotic twins
duplication of primitive streak in monozygotic twins
- shared fetal membranes
2. documented in mouse and chicken (1% of human monozygotic twins)
what are the rules for classification of twins
genetic identity
degree of separation
symmetry of separation
genetic identity
dizygotic (2 genotypes) vs. monozygotic (identical genotypes)
degree of separation can be
free or conjoined
free
embryo/fetuses completely separated from each other
conjoined
embryo/fetuses attached to one another
conjoined twins are ____ in origin
monozygotic in origin and represent an incomplete separation between the 2
symmetry of separation can be classified as
symmetrical or asymmetrical
symmetrical
equal size/body parts, duplication of body axis
asymmetrical
unequal size, extra body parts without duplication of body axis
freemartins
extraembryonic vessels of twins anastomose allowing exchange of fluid between fetuses before sexual differentiation;
one twin is XX (female) and other is XY (male)
female fetus affected by hormones (antimullerian duct hormone and testosterone) of male twin fetuses
most common intersex condition is a
pseudohermaphodite (sterile)
masculinization of female tract in pseudohermaphodite)
external genitalia, female but reduced in size except for enlarged clitoris (small vulva, enlarged clitoris, encreased anogenital distance)
gonads look like testes and form seminiferous tubules, produce no sperm, cell are genetically X
short, blind-ended vagina (1-4 week old calves; normal 13-15 cm; freemartin 5-6 cm)
cervix absent
what are free symmetrical monozygotic twins
genetically identical twins (derived from single zygote)
not attached to each other in any way
separation typically between,
2 cell blastomere and blastocyst stage, generally 2 separate sets of fetal membranes form
free assymetrical twins may be
monozygotic or dizygotic
free asymmetrical twins are not _____
physically connected by body parts, but vascular systems interconnected
in free asymmetrical twins, what are the twins like
one twin is normal, other is underdeveloped
terms to describe undeveloped twin
amorphout globusus
anidian fetus
acaardiac fetus
holocardius
why does the undeveloped twin survive
because of connection to blood supply
characteristics of undeveloped twin
non-distinct body form, disorganized collection of tissues (skin, muscle, teeth, rudimentary GI organs)
human
twin reveresed arterial perfusion or TRAP sequence
twin to twin transfusion
exchange of blood between free twins
undeveloped twin heart
underdeveloped and not fully
circulation od undeveloped twin is driven by
normal twin
in free asymmetrical twins, the normal twin is
pumping blood for 2fetuses
this can lead to
heart failure in fetal life and intrauterine death (human mortality rat is 50%)
conjoined symmetrical twins
incomplete separation of monozygotic twins, typically in primitive streak stage, duplication of body axis
where is primitive streak duplicated?
at cranial or caudal end, but fails to divde completely
diplopagus
nearly complete, connected by a small amount of tissue
dicephalus
2 heads
diprosopus
2 tails
tetrabrachius
2 pair of thoracic limbs
tetrasccelus
2 pair pelvic limbs
conjoined assymetrical twins
incomplete separation of monozygotic twins, marked unequal size or without duplication of body axis
extra body parts are of the conjoined fetus are typically
smaller and misshapen
mummified fetus is not necessarily a
twin fetus phenomenon
mummified fetus occurs when
a normal fetus arrests in fetal development and dies
the mummified fetus becomes ____ in uterus
dehydrated and shrunken
a mummified species should not be mistaken for ______ in twin pregnancies
an underdeveloped, free symmetrical twin
